Holding on to a past hurt is common. Most of us have done it at least once. We get our feelings hurt or feel we have been shamed or wronged. We then hold onto that hurt and the anger at whomever we feel was responsible. We may also go so far as to treat the other person (or persons) with disdain for as long as we can stand it. It could be for a long time. It could also vary from being an irritation to full rage that we embrace whenever we think of the pain.
Mark Twain indicated that "Anger is an acid that can do more harm to the vessel in which it is stored than to anything on which it is poured." The notion is that we are the ones who continue to be harmed because we hold onto anger and hurt. We let the past take the place of the potential joy we could have in our lives. I have known people who lived their lives looking for ways to hurt another person as much as they felt they were injured. What if the offending person's deeds and motives were misunderstood? All of that time and energy are lost in creating even more pain and suffering.
Whole nations and cultures have allowed hatred to fester and grow, sometimes for generations. The offense has been long forgotten, but the anger toward that offending group persists. It takes a lot of energy to maintain that amount of anger and resentment. Holding onto the past hurt also allows the "acid" to harm our own lives.
Probably one of the most difficult things to do is to release the resentment toward someone who we feel has injured us. It almost feels like we suffered for naught if we let go of the anger. However, if we don't release the "acid," we get mired in resentment that will continue to diminish our own lives. The most courageous step we can make is to let go and move on.